Hormones such as leptin and ghrelin can rapidly rewire hypothalamic feeding circuits when injected into rodent brains. These experimental manipulations suggest that the hypothalamus might reorganize continually in adulthood to integrate the metabolic status of the whole body. In this study, we examined whether hypothalamic plasticity occurs in naive animals according to their nutritional conditions. For this purpose, we fed mice with a short-term high-fat diet (HFD) and assessed brain remodeling through its molecular and functional signature. We found that HFD for 3 d rewired the hypothalamic arcuate nucleus, increasing the anorexigenic tone due to activated pro-opiomelanocortin (POMC) neurons. We identified the polysialic acid molecule (PSA) as a mediator of the diet-induced rewiring of arcuate POMC. Moreover, local pharmacological inhibition and genetic disruption of the PSA signaling limits the behavioral and metabolic adaptation to HFD, as treated mice failed to normalize energy intake and showed increased body weight gain after the HFD challenge. Altogether, these findings reveal the existence of physiological hypothalamic rewiring involved in the homeostatic response to dietary fat. Furthermore, defects in the hypothalamic plasticitydriven adaptive response to HFD are obesogenic and could be involved in the development of metabolic diseases.

2008). In addition, hypothalamic plasticity appears to be a widely conserved process found in frogs, birds, rodents, and primates. The involvement of hypothalamic plasticity in the control of whole-body energy homeostasis emerged as a new concept in 2004 (Pinto et al., 2004). This process seems to be essential and its impairment could contribute to obesity. In laboratory animals, rapid rewiring of the hypothalamus can be achieved by using various experimental procedures, including fasting and hormone treatments with exogenous hormones such as leptin and ghrelin (Pinto et al., 2004; Sternson et al., 2005; Andrews et al., 2008; Yang et al., 2011). Such manipulations produce marked changes in feeding behavior, which are probably triggered, at least in part, by the stimulated hormone-dependent reorganization of synapses in specific hypothalamic neurons (Pinto et al., 2004; Sternson et al., 2005; Andrews et al., 2008; Yang et al., 2011). Nevertheless, whether hypothalamic plasticity could play a role in the regulation of food intake in naive animals according to changes in their nutritional conditions is still unknown. To address this issue, we explored hypothalamic plasticity in adult mice fed a high-fat diet (HFD) for 1 week.

Materials and Methods Animals. Protocols that included the manipulation of animals were reviewed by our local ethics board and were in strict accordance with European Community guidelines (directive 86/906). Experiments were

performed with 2-month-old male C57BL/6JOla mice from Harlan Laboratories. The mice were housed individually, and fed a standard pelletized commercial chow diet (A04; Safe) for 1 week after arrival. After acclimatization, they were fed either the same standard diet (STD) or a customized highly palatable high-fat diet (Safe). The characteristics of the diets are given in Table 1. The change of diet was made at 9:00 A.M. and both the standard and high-fat diet were renewed daily at 9:00 A.M. The mice had ad libitum access to food and water. Food consumption and body weight (BW) were measured daily. For metabolic studies by indirect calorimetry, the mice were housed in individual air-tight cages and gas exchanges were monitored using an air analyzer system (Oxylet; BIOSEB). For tissue collection, the mice were killed between 9:30 A.M. and 12:00 P.M. Some experiments used 8- to 10-week-old male transgenic homozygous knockout PST-1 mice. These mice had been generated by using targeted mutations in the ST8SiaIV gene (Eckhardt et al., 2000). Bilateral injection into the hypothalamus. The mice were placed in a stereotaxic frame (David Kopf Instruments) under anesthesia with 0.5–2% isoflurane constant gas inhalation (Forene; Abbott Laboratories). After dermal disinfection with Vetadine solution (Vetoquinol), the skin and cranial muscles were incised and the skull was exposed. A small hole was drilled and a needle was inserted to target each ventromedial hypothalamus nuclei successively using the following coordinates: ⫺1.5 mm posterior to the bregma, ⫾0.4 mm lateral to the sagittal suture, and ⫺5.6 mm below the skull surface. Endoneuraminidase N (EndoN; AbCys) or artificial CSF (aCSF; Tocris Bioscience) was injected through a 34 ga blunt needle mounted on a 10 ␮l syringe (NanoFil device from WPI) controlled by a micropump (UMP2 from WPI). A volume of 400 nl per side of both solutions was delivered into the brain parenchyma at rate of 100 nl/min. EndoN-treated mice received 0.28 U per side. After injection, the needle was maintained for a further 3 min to avoid back leakage. Finally, the skin was sutured using cyanoacrylate glue. After surgery, the animals were kept under controlled temperature and rehydrated with intraperitoneal injections of physiological fluid. The mice were then housed individually and were allowed 3 d for recovery before the experiment, i.e., before the diet change. Hypothalamus dissection for qPCR analysis. Once the mice had been killed, their brains were quickly removed and immersed for 10 min in 2 ml of ice-cold preservative medium (200 mM sucrose, 28 mM NaHCO3, 2.5 mM KCl, 7 mM MgCl2, 1.25 mM NaH2PO4, 0.5 mM CaCl2, 1 mM L-ascorbate, and 8 mM D-glucose, pH 7.4) complemented with 10% RNAlater (Ambion). After incubation, the brains were mounted on a specimen plate using cyanoacrylate adhesive. The brains were then sliced using a vibratory microtome in a bath containing the chilled preservative medium with RNAlater. Five 500-␮m-thick serial coronal sections per brain were selected with the assistance of a mouse brain atlas, approximately from ⫺3.0 to ⫺0.5 to bregma. The slices were individually placed onto dishes containing 500 ␮l of RNAlater. For the microdissection of the hypothalamus nuclei, each slice was placed on a 6% agarose bloc, covered with 50 ␮l of RNAlater, and dissected under stereomicroscope and cold-light illumination, using a scalpel and sharp forceps. The arcuate nucleus (ARC), lateral hypothalamus area, and paraventricular nucleus were collected from two or three specific slices of the five, depending on their anteroposterior anatomical position. During the dissection, the harvested samples were placed on ice in 2 ml centrifuge tubes containing 50 ␮l of RNAlater. After removal of the RNAlater, the samples were finally stored at ⫺80°C.

RNA extraction and processing. The tissues were lysed and homogenized in 300 ␮l of lysis buffer (RLT Buffer, Qiagen) using the TissueLyser system (Qiagen) and 5 mm stainless steel beads (Qiagen). Total RNA was isolated on spin columns with silica-based membranes (RNeasy Mini Kit, Qiagen), following the manufacturer’s instructions. DNA digestion was done directly on the columns. RNA was eluted with 30 ␮l of H2O. Aliquots of each extract (1 ␮l) were checked for RNA concentration, purity and integrity with the Experion electrophoresis system (Bio-Rad) and the Experion RNA StdSens Analysis Kit (Bio-Rad). Total RNAs were then stored at ⫺80°C. A small amount of purified RNAs (0.2 ␮g) was reversetranscribed in 20 ␮l of mixture using the High-Capacity cDNA Archive Kit (Qiagen), as indicated by the manufacturer. Synthesized cDNA were then stored at ⫺20°C. qPCR analysis by TaqMan low-density array. The low-density array (LDA) is a 384-well micro-fluidic card on which 384 simultaneous realtime PCRs can be performed (Applied Biosystems). Each custom card was configured as 8-sample loading lines containing 48 reaction chambers. Gene-specific exon-spanning primers and TaqMan probes were factory-designed and embedded in each well. Analysis of one hypothalamic sample consisted in loading 100 ␮l of reaction mixture into one port of the LDA. The mixture comprised 15 ␮l of synthesized cDNA (corresponding to 150 ng of RNA), 50 ␮l of TaqMan Gene Expression Master Mix (Applied Biosystems), and 35 ␮l of water. After loading, the LDA was sealed and centrifuged twice for 2 min at 280 ⫻ g. The LDA was placed in the 384-well block module of a thermal cycler (model 7900HT Fast Real-Time PCR system, Applied Biosystems). The PCR conditions were 2 min at 50°C and 10 min at 95°C, followed by 40 cycles of 15 s at 95°C and 1 min at 60°C. Raw fluorescence data were collected through the PCR using the SDS 2.3 software (Applied Biosystems), which further generated threshold cycles Ct with automatic determination of both baseline and threshold. After SDS software-assisted filtering of assays to discriminate aberrant PCR runs, the assays per hypothalamic area were: n ⫽ 9 –12 for STD, n ⫽ 6 –7 for HFD D1 (day 1), n ⫽ 6 –7 for HFD D3, and n ⫽ 5– 6 for HFD D8. The data were then analyzed with RQ Manager 1.2 software (Applied Biosystems) for relative quantitation. Relative quantitation of gene expression (RQ) was based on the comparative Ct method using the equation RQ ⫽ 2 ⫺⌬⌬Ct, where ⌬⌬Ct for one gene target was its own Ct variation subtracted from a calibrator sample and normalized with an endogenous control. Precisely, polr2a was defined as the endogenous control after analysis of the most stable housekeeping gene using geNorm freeware, and one STD sample was arbitrarily chosen as a calibrator. Graphic representation of results was manually designed to assign one color for a 10% increment of gene expression relative to the STD group. Significant variation was noted with an asterisk. Immunohistochemistry. The mice were anesthetized by intraperitoneal injection of ketamine/xylazine mix and then perfused intracardially with 4% paraformaldehyde solution. The brains were removed, postfixed at 4°C overnight, cryoprotected with 30% sucrose for 2 d at 4°C, frozen in isopentane at ⫺60°C, and finally stored at ⫺80°C until use. The hypothalamus was cut into 30 ␮m serial sections with a cryostat (Leica). Five sections of the 15–18 harvested sections containing the arcuate nucleus were treated for immunohistochemistry. The sections were first blocked for 3– 4 h and then incubated overnight at 4°C with anti-PSA (polysialic acid molecule) antibody (1:6000, #AbC0019, EuroBio). After washing, the sections were then incubated in Alexa546-conjugated goat anti-mouse IgM (Invitrogen, 1:400) for 2 h at room temperature. After further washes, the sections were finally held with mounting medium and a coverslip. Image acquisition and analysis. For densitometric analysis, immunolabeled sections were viewed on a confocal microscope (Leica SP2) under the 40⫻ oil-immersion objective. Images of an immunostaining run (one animal of each condition: STD, HFD 1 d, HFD 3 d, HFD 8 d) were acquired with the same parameters (561 nm laser power, gain and offset of the photomultiplier). By using ImageJ software, the intensity of PSA labeling per section was bilaterally quantified on a selected area corresponding to the arcuate nucleus. Labeling intensity was measured on 5 sections per animal. The dentate gyrus was chosen as the control area. For the large-field study, sections were observed using an upright light microscope Axio Imager 2 (Zeiss) equipped with a motorized stage. Entire brain sections were scanned automatically under the 40⫻ objective.

with picrotoxin (100 ␮M) to block GABAergic IPSCs. For IPSC recordings, pipettes were filled with a cesium-chloride solution containing (in mM): 140 CsCl, 3.6 NaCl, 1 MgCl2, 10 HEPES, 0.1 Na4EGTA, 4 Mg-ATP, 0.25 Na-GTP (290 mOsm, pH 7.3). A mixture of 6-cyano-7-nitroquinoxaline-2,3-dione (20 ␮M) and (2R)-amino-5-phosphonovaleric acid (50 ␮M) was added to the extracellular medium to block glutamatergic currents. Miniature EPSCs and IPSCs were isolated and monitored by adding tetrodotoxin (TTX, 500 nM) to the extracellular solution. The recordings were made using an Axopatch 1D amplifier, digitized using the Digidata 1320A interface and acquired using pClamp 9.2 software (Molecular Devices). The pipettes and cell capacitances were fully compensated. Access resistance was monitored over the course of the recordings. Cells were excluded if the access resistance was ⬎35 M⍀ and/or increased significantly (⬎20% of change) during the experiment. Cells were voltage clamped at ⫺60 mV, and sampled at a frequency of 10 kHz. Currents were recorded for at least 5 min. EPSC and IPSC frequency was calculated over a period of at least 150 s using Clampfit software (Molecular Devices). Statistical analysis. All data are expressed as means. Error bars indicate SEM. Multiple comparisons of groups were performed by a one-way ANOVA using Prism 4.0 software (GraphPad Software). Post hoc analyses were used when main effects reached significance without any mathematical correction. Before analysis, Bartlett’s and Shapiro–Wilk’s tests were applied to check equality of variances and to evaluate the normality of distribution, respectively. Bonferroni’s, Newman–Keuls’, or Dunnett’s tests were used to compare groups. Student’s t test was used when only two groups (STD vs HFD) were studied. When variances were significantly different, the Mann–Whitney test was applied. Significant difference was noted on the graphic representation when p value was ⬍0.05, 0.01, or 0.001.

D1, HFD D3, and HFD D8, determined using one-way ANOVA and Dunnett’s post hoc test against STD), and adiposity was increased on the third day (fat mass STD: 0.63 ⫾ 0.13; fat mass HFD D3: 1.08 ⫾ 0.11 g; Fig. 1 E; p ⬍ 0.05 for HFD D3, p ⬍ 0.001 for HFD D8, determined using the Mann–Whitney test against STD), suggesting appropriate nutrient channeling and efficient storage of the energy overload. We concluded that the mice rapidly adapted their eating behavior and metabolism to the hypercaloric nutritional condition. Indeed, this model constitutes a paradigm of homeostatic control in response to dietary fat, as suggested by Butler et al. (2001). Molecular signature of brain plasticity is induced in the hypothalamus after HFD feeding The adaptive response to sustained external stimuli or durable physiological modification can be linked to tissue rearrangement in specific brain areas (Hu¨bener and Bonhoeffer, 2010; McEwen, 2010). These activity-dependent modifications are based on the co-

Benani et al. • PSA-Dependent Diet-Induced Hypothalamic Plasticity

J. Neurosci., August 29, 2012 • 32(35):11970 –11979 • 11975

variation was detected in the dentate gyrus of the hippocampus, the most frequently described brain area able to undergo remodeling. Novelty is not sufficient to induce PSA in the arcuate nucleus The molecular signature of plasticity that is detected in the hypothalamus after HFD feeding could be induced by the change in food composition or merely by the stress linked to the novelty. We therefore tested whether the replacement of food was sufficient to stimulate PSA expression in the arcuate nucleus. A standard diet was replaced by an isolipidic isocaloric control (CTRL) diet, whose composition was similar but which differed in appearance, in texture, and probably in flavor (Fig. 3 A, B). Introduction of the CTRL diet did not modify the energy intake of mice (Fig. 3C). One day after feeding with the STD, HFD or CTRL diet, PSA abundance was measured in protein extracts from arcuate nuclei by Western blot (Fig. 3D). Again, Figure 4. Arcuate POMC neurons of mice fed an HFD for 3 d are rewired. Postsynaptic excitatory (A) and inhibitory (B) currents the HFD for 1 d increased PSA levels in the in GFP-tagged POMC neurons from standard mice and mice fed an HFD for 3 d were recorded in a whole-cell voltage-clamp arcuate nucleus (Fig. 3E; p ⬍ 0.05, deterconfiguration (holding potential at ⫺60 mV). Representative sample traces (30 s) are given in each panel. Frequencies of spon- mined using the Mann–Whitney test taneous and miniature currents were calculated before and after adding TTX, respectively. Data are means ⫾ SEM. Groups were against STD). By contrast, the CTRL diet did not modify the PSA level. Moreover, compared using Student’s t test. Significant difference at *p ⬍ 0.05. after 6 d on the STD diet, 24 h of reexposure to the HFD still increased energy inordinated regulation of mRNA expression (Bramham and Wells, take (Fig. 3C; p ⬍ 0.001, determined using the Mann–Whitney 2007) and protein synthesis (Cajigas et al., 2010). To determine test against STD), and increased the PSA level in arcuate biopsies whether HFD feeding induces plasticity within the hypothalamus, (Fig. 3E; p ⬍ 0.05, determined using the Mann–Whitney test we measured the relative expression of several permissive factors of against STD). Together, these results indicate that novelty in food brain plasticity in dissected hypothalamic areas (i.e., arcuate nucleus, conditions alone is not sufficient to increase PSA levels in the paraventricular nucleus and lateral hypothalamus) using lowhypothalamus. density arrays. Screening was performed after 1, 3, and 8 d of HFD feeding. Compared with standard-diet-fed mice, mRNA abundance HFD feeding for 3 d induces arcuate POMC neuron rewiring of a cluster of plasticity markers (n ⫽ 10) was significantly increased The melanocortin system in the CNS plays a fundamental role in in biopsies from HFD-fed mice (from 10 to 40%) (Fig. 2A; statistical the regulation of energy homeostasis by producing anorexigenic significance determined using one-way ANOVA and Bonferroni effects when stimulated (Cone, 2005; Berthoud and Morrison, post hoc test, or using Mann–Whitney test when variances were un2008). The response controlled by POMC neurons is actually equal). This change was rapid and transient, occurring after 1 and 3 d defined by their intrinsic neuronal activity and synaptic inputs. of HFD feeding, and was mostly located in the arcuate nucleus. We Interestingly, these inputs can be rapidly rewired (Pinto et al., found upregulation of ncam1, nrp1, tnc, snap25, syp, syt4, suggesting 2004; Sternson et al., 2005). To investigate whether the HFD that HFD modified cell interactions and induced synaptogenesis. By challenge could affect presynaptic inputs on arcuate POMC neucontrast, expression of gap43, serpine1, mmp9, or plau was not afrons, we recorded the frequencies of spontaneous and miniature fected suggesting no change in axonal sprouting. Changes in cell postsynaptic currents (sPSCs and mPSCs) in arcuate GFP-tagged interactions can be mediated by the polysialic acid (PSA) molePOMC neurons held in the whole-cell voltage-clamp configuracule (Rutishauser, 2008). PSA is a cell-surface glycan with a large tion, using brain slices from mice fed either a standard diet or hydrated volume that modulates distances between cells. Basically, HFD for 3 d. We found an increase in the frequency of spontathe attachment of PSA to membrane proteins, such as the neural neous EPSCs (sEPSCs) on arcuate POMC neurons in HFD mice cell-adhesion molecule (NCAM), promotes synaptic reorganization (STD: 2.12 ⫾ 0.55 Hz; HFD: 4.63 ⫾ 0.87 Hz; Fig. 4 A; p ⬍ 0.05, and other plasticity-related events by weakening cell-to-cell interacdetermined using t test). By contrast, the frequency of spontanetions. We therefore examined levels of PSA in adult brains from ous IPSCs was decreased in HFD fed mice (STD: 1.47 ⫾ 0.17 Hz; standard- and HFD-fed mice. The brains were fixed, sectioned and HFD: 0.86 ⫾ 0.17 Hz; Fig. 4 B; p ⬍ 0.05, determined using t test). analyzed for the abundance of PSA by immunohistochemistry. A We also quantified the frequency of mPSCs arising from spontatwofold increase in arcuate PSA immunoreactivity was calculated neous vesicle fusion by using TTX to block all action potentialfrom optical sections acquired by confocal laser scanning microdriven PSCs. The quantification of mPSC frequency gives an scope (Fig. 2B; F(3,24) ⫽ 4.67; p ⬍ 0.05 for HFD D1, determined indirect estimation of the number of synapses onto postsynaptic using one-way ANOVA and Bonferroni post hoc test), whereas no neurons. Miniature EPSC (mEPSCs) frequency was increased

2012), hypothalamic cell renewal might also contribute to the homeostatic response to dietary fat. Therefore the homeostatic control of energy balance is surely consolidated by a combination of several plasticityrelated processes from rapid pharmacological to slower morphological changes. Unfortunately, all of these brain safety mechanisms are obviously overtaken when the calorific pressure is sustained. Interestingly, the molecular screening of plastic events in the hypothalamus of HFD mice using low-density arrays sugFigure 6. PST-1 enzyme deficiency impairs the homeostatic control of energy intake after HFD introduction. A, Representative PSA gests a brief and arcuate-specific modifiimmunostaining in arcuate nucleus of PST-1 ⫹/⫹ and PST-1 ⫺/⫺ mice fed an HFD for 1 d. B, Energy intake of PST-1 ⫹/⫹ and PST-1 ⫺/⫺ cation in cell interactions. Although the mice fed an HFD for 5 d. Data are means ⫾ SEM. n ⫽ 6 PST-1 ⫹/⫹ mice and n ⫽ 12 PST-1 ⫺/⫺ mice. Groups were compared using melanocortin system is a widespread neuStudent’s t test. ronal network, diet-induced synaptic plasticity seems to affect arcuate, probably first-order, neurons only. Therefore, d/g; Fig. 6 B). Moreover, energy intake of wild-type and mutant downstream targets of arcuate POMC or AgRP neurons such as mice increased similarly on day 1 after HFD introduction (0,73 ⫾ paraventricular MC4R-positive neurons, which are crucial in the 0.03 and 0,75 ⫾ 0.05 kcal/d/g, respectively). However, in mutant homeostatic response to dietary fat (Butler et al., 2001), could mice the return to basal level took 1 d longer than in wild-type mice. Consequently, the cumulative energy intake was increased relay the increased anorexigenic tone from the arcuate nucleus by 8.6% in mutant mice fed for 5 d with HFD compared with without particular synaptic remodeling. their wild-type littermates (wild-type: 2.95 ⫾ 0.09 kcal/g, KO: In this article, we report a PSA-dependent control of body 3.20 ⫾ 0.07 kcal/g; p ⬍ 0.05, determined using t test). Body weight. The obesogenic effect of EndoN could be linked to an weight gain of HFD-fed mutant mice was not significantly inalteration in food intake regulation, but an effect on energy excreased (wild-type: 0.38 ⫾ 0.22 g, KO: 0.69 ⫾ 0.14 g), probably penditure is not excluded either. Indeed, POMC neurons also exposure to the HFD was too short. These results demonstrate govern “facultative” thermogenesis, which burns off excess calothat the PST1 enzyme plays a role in the behavioral adaptation to ries during times of plenty. On the other hand, the restoration of HFD. glucose tolerance is not EndoN sensitive, suggesting that PSAdependent neuronal rewiring is not crucial to maintain glucose Discussion homeostasis. Thus, PSA-dependent hypothalamic plasticity apHypothalamic plasticity could be defined as an adaptive process pears to act on specific physiological responses such as the reguaimed at integrating changes in environmental conditions and lation of food intake. This was unexpected, given the broad physiological states (Oliet, 2002; Prevot, 2002; Ebling and Barrett, spectrum of action of the melanocortin system on peripheral 2008). Here, we report that a change of diet is another situation metabolism (Mountjoy, 2010). However, to our knowledge, horleading to hypothalamic neuronal network rewiring. Indeed, permonal stimulation of POMC neuron rewiring does not alter glusistent fat ingestion increases the frequency of miniature excitcose homeostasis either. atory postsynapic currents in POMC neurons, which strongly It appears that synaptic activity on POMC neurons promotes suggests a synaptic reorganization on these cells (Pinto et al., a homeostatic response to dietary fat ingestion, i.e., the progres2004). In this study, we did not find the causative link between sive reduction of food intake over a week. The change in hypothese electrophysiological data and the modification of food inthalamic neuron connectivity persists for several months even take after HFD introduction, which requires inducible and tarthough the HFD continues (Horvath et al., 2010). However, the geted inhibition of PSA-mediated synaptogenesis in POMC cells. different outcomes in short- and long-term exposure to HFD However, (1) PSA overexpression precedes behavioral changes, could be linked to leptin resistance in diet-induced obese ani(2) both, HFD-induced POMC rewiring and the progressive resmals, a situation in which elevated leptin levels no longer sustain toration of energy intake are EndoN sensitive, and (3) anorexiPOMC neuronal firing (Cowley et al., 2001; Enriori et al., 2007; genic POMC neurons are thought to be involved in adaptive Diano et al., 2011). homeostatic processes that maintain energy homeostasis (Cone, We identified PSA as a downstream actor required for the 2005). As a result, the HFD-induced PSA-dependent POMC rediet-induced rewiring of POMC neurons. Polysialylation is a wiring could explain the homeostatic response to dietary fat. ubiquitous mechanism found in several hypothalamic processes Although the role of the melanocortin system in the adaptathat involve modifications in cell interactions (Theodosis et al., tion of food intake in response to variations in nutritional con1991). Therefore, it is probably not specific to the diet-dependent ditions has been already proposed (Butler et al., 2001; Pillot et al., synaptic plasticity of arcuate POMC neurons. Rather, it could be 2011), synaptic reorganization on arcuate POMC neurons now considered a common permissive process that might be recruited has to be considered a key component in the physiological feedin other previously described hormone-dependent rewiring of back. Nevertheless, we cannot exclude either a synergic effect due these neurons (Pinto et al., 2004; Gao et al., 2007; Gyengesi et al., to mirror synaptic rewiring of orexigenic cells, NPY/AgRP neurons 2010; Yang et al., 2011). In addition, according to the transcripfor instance. Additional defenses against metabolic imbalance also tomic assay, other regulators of dynamic cell interactions and involve nonsynaptic mechanisms, such as the HFD-stimulated upsynaptogenesis might also be involved in diet-induced hypotharegulation of POMC expression (Ziotopoulou et al., 2000). As cell lamic plasticity. Indeed, syndecan-3 and synaptotagmin-4 are turn-over in feeding circuits is inhibited in obese mice (McNay et al.,

11978 • J. Neurosci., August 29, 2012 • 32(35):11970 –11979

promising targets to control obesity and related diseases (Strader et al., 2004; Zhang et al., 2011). Biological mediators that promote diet-induced hypothalamic plasticity have not been investigated in this study, but one can easily speculate that metabolic hormones, which act on the energy metabolism through their neurotrophic properties, might be involved in this physiological mechanism (Pinto et al., 2004; Abizaid et al., 2006; Coppola et al., 2007; Andrews et al., 2008; Chiu and Cline, 2010; Yang et al., 2011). Leptin is one of the putative candidates. Blood leptin levels promptly increased after 3 d of HFD (Wang et al., 2001). This is consistent with the fat mass expansion detected in our model. Therefore, leptin could promote the synaptic reorganization of POMC neurons to inhibit food intake in addition to its direct stimulating effect on POMC neuronal activity. The contribution of nutrients themselves and/or their metabolites in this process should be considered too. For instance, dietary fat-derived endocannabinoids are major components of the gut-brain axis and can engender synaptic alterations in the brain (Crosby et al., 2011; Lafourcade et al., 2011; Bermudez-Silva et al., 2012). On the other hand, it seems that stress-related signals are not involved in diet-induced hypothalamic plasticity because familiar conditions, like reexposure to HFD, still produced upregulation of the plasticity marker PSA, whereas food novelty was not sufficient to induce this response. Since hypothalamic plasticity appears to be a widely conserved process (Peinado et al., 2002; Pinto et al., 2004; Ebling and Barrett, 2008; Appelbaum et al., 2010; Baroncini et al., 2010), dietinduced hypothalamic plasticity could be present in humans as well. Indeed, haploinsufficiency of BDNF, the typical permissive factor of brain plasticity, is associated with childhood-onset obesity (Han et al., 2008). Furthermore, two recent genome-wide association studies of large human cohorts have reported a strong association between a high body-mass index and polymorphic loci whose neighboring genes are highly expressed in the brain and appear to be involved in neuronal development and/or activity (Thorleifsson et al., 2009; Willer et al., 2009). Thus, these studies highlight the crucial role that brain plasticity may play in regulating food intake and energy homeostasis in humans as well. In conclusion, our findings bring new insights into the regulation of food intake. We show that the melanocortin system quickly adapts to the ingested food. Diet-induced rewiring of POMC neurons produces effects on energy intake. Inability to initiate diet-induced hypothalamic plasticity is obesogenic and could therefore be a new factor in the etiology of metabolic diseases.